Prepare the Dumpling Dough:

1. Mix the Dough: In a large bowl, combine the flour and salt. Gradually add the water, a little at a time, stirring constantly. Once the dough begins to come together, add the oil and continue mixing until you form a smooth, elastic dough. If the dough is too sticky, add a little more flour. If it’s too dry, add a small splash of water.
2. Rest the Dough: Cover the dough with a damp cloth or plastic wrap and let it rest for at least 15 minutes. This helps the dough become more pliable and easier to roll out.

Prepare the Dumpling Filling:

3. Cook the Filling: In a skillet, heat the sesame oil over medium heat. Add the chopped onion and minced garlic. Sauté for 2-3 minutes until fragrant and translucent. Add the ground chicken or beef to the skillet and cook, breaking it up with a spoon, until the meat is fully cooked through.
4. Season the Filling: Once the meat is cooked, add the soy sauce, rice vinegar, grated ginger, brown sugar, and ground black pepper. Stir well and cook for another 1-2 minutes. If the filling is too dry, add a tablespoon or two of water. Remove from heat and let the filling cool to room temperature.
5. Add Vegetables (Optional): If you’re using cabbage or spinach, add it to the mixture now. Make sure it’s finely chopped to fit into the dumplings easily.

Assemble the Dumplings:

6. Roll Out the Dough: Divide the rested dough into small, equal portions (about 20-25 pieces). Roll each piece into a small ball, then use a rolling pin to flatten it into a thin round disc (about 3 inches in diameter).
7. Add the Filling: Place a spoonful of the cooled filling in the center of each dough disc. Be careful not to overstuff the dumplings, as this can cause them to burst open while frying.
8. Seal the Dumplings: Fold the dough in half over the filling to form a half-moon shape. Pinch the edges together tightly, ensuring that the filling is sealed inside. If needed, use a fork to press the edges together and create a decorative pattern.

Cook the Dumplings:

9. Heat the Oil: In a large non-stick skillet, heat the vegetable oil over medium-high heat. Once hot, add the dumplings to the pan, making sure they don’t touch each other.
10. Pan-Fry: Fry the dumplings for 2-3 minutes on each side, until they are golden brown and crispy. The goal is to get a nice, crunchy texture on the bottom and edges.
11. Smash the Dumplings: Once the dumplings are golden and crispy on both sides, use a spatula to gently smash them down. Press lightly to flatten them, creating a crispy, crunchy texture across the surface.
12. Steam the Dumplings: After smashing, add a small amount of water (about ¼ cup) to the pan and cover immediately with a lid. Let the dumplings steam for another 2-3 minutes until fully cooked through and the water has evaporated.
13. Serve: Once done, remove the dumplings from the skillet and sprinkle with a pinch of sea salt for added flavor, if desired.